How to read last 5 lines of a .txt file into java

Question

I have a text file that consists of several entries such as:

hello
there
my
name
is
JoeBloggs

How would I read the last five entries in des

Answer 1

Follow This Code To Improve Core Java Logic By Using Collectios.

import java.io.FileNotFoundException;
    import java.io.FileReader;
    import java.util.ArrayList;
    import java.util.Collection;
    import java.util.Collections;
    import java.util.Scanner;

    public class REVERSE {
        public static void main(String[] args) {
            ArrayList<String> al = new ArrayList<String>();
            try {
                Scanner sc = new Scanner(new FileReader("input.txt"));
                while (sc.hasNextLine()) {
                    al.add(sc.nextLine());
                }
                System.out.println(al.get(0));
                System.out.println(al.get(1));
                System.out.println(al.get(2));
                System.out.println(al.get(3));
                System.out.println(al.get(4));

                Collections.reverse(al);
                /*
                 * for (String s : al) { System.out.println(s); }
                 */
                System.out.println(al.get(0));
                System.out.println(al.get(1));
                System.out.println(al.get(2));
                System.out.println(al.get(3));
                System.out.println(al.get(4));
                /*
                 * for (int i = 0; i < al.size(); i++) {
                 * System.out.println(al.get(i)); }
                 */
            } catch (FileNotFoundException e) {

            }

        }
    }

Answer 2

You can add the lines to a List, e.g. a LinkedList. When the list has more than five lines, remove the first/last.

List<String> lines = new LinkedList<String>();
for(String tmp; (tmp = br.readLine()) != null;) 
    if (lines.add(tmp) && lines.size() > 5) 
        lines.remove(0);

Answer 3

we can use MemoryMappedFile for printing last 5 lines:

private static void printByMemoryMappedFile(File file) throws FileNotFoundException, IOException{
        FileInputStream fileInputStream=new FileInputStream(file);
        FileChannel channel=fileInputStream.getChannel();
        ByteBuffer buffer=channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());
        buffer.position((int)channel.size());
        int count=0;
        StringBuilder builder=new StringBuilder();
        for(long i=channel.size()-1;i>=0;i--){
            char c=(char)buffer.get((int)i);
            builder.append(c);
            if(c=='\n'){
                if(count==5)break;
                count++;
                builder.reverse();
                System.out.println(builder.toString());
                builder=null;
                builder=new StringBuilder();
            }
        }
        channel.close();
    }

RandomAccessFile to print last 5 lines:

private static void printByRandomAcessFile(File file) throws FileNotFoundException, IOException{
        RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
        int lines = 0;
        StringBuilder builder = new StringBuilder();
        long length = file.length();
        length--;
        randomAccessFile.seek(length);
        for(long seek = length; seek >= 0; --seek){
            randomAccessFile.seek(seek);
            char c = (char)randomAccessFile.read();
            builder.append(c);
            if(c == '\n'){
                builder = builder.reverse();
                System.out.println(builder.toString());
                lines++;
                builder = null;
                builder = new StringBuilder();
                if (lines == 5){
                    break;
                }
            }

        }
    }